Polyesters and polyamides are widely used in a variety of applications, such as in apparel, fibers, films, and plastics. This is, in part, due to their excellent mechanical and physical properties. Such polyester fibers have become the first choice for apparel and are used in textile applications such as manufacture of trousers, skirts, dresses, suits, jackets, blouses, outdoor clothing, and carpets. Polyester fibers have good moisture transport, dry quickly, high tenacity, dyeability, easy handling properties, dye fastness, innovative designs, and different shades. Incorporation of cationic sulfonated molecules into polyesters imparts improved dyeing with a wide range of dyes. Where polyester fibers are to be used, chemical modification of the underlying polyester with functional groups may be required. For example, the dyeability of polyethylene terephthalate is much greater when functionalized with certain groups as compared to polyethylene terephthalate without functionalization. Chemical modification of polyethylene terephthalate to introduce functional groups, permits a substantial change in its dyeability without a significant change in its physiomechanical properties.
Chemically modified, cationic-free dyeable polyester (CFDP) is produced by adding certain additives, such as polyethylene glycol (PEG), adipic acid, or azillic acid to form block polymers with polyesters. Such polyesters exhibit good dyeability at 100° C., as well as good physical properties and tensile strengths that are comparable with the normal polyester. The glass transition temperature (Tg) of such fibers is about 100° C. This is lower than normal polyester, thus leading to a higher segmental mobility. This, in turn, increases the rate of dye diffusion into fibers at a lower temperature, providing for dyeing of deep shades, even in the absence of carriers. Carrier-free dyeable polyester, however, is associated with many problems. Illustrative problems include homogeneous dyeing, color retention, and wash fastness of dyeing. Wash fastness of the dyeing is slightly lower for these fibers than normal polyester fibers because of the fiber structure. Thus, appropriate instructions should be given to consumers to wash CFDP products at temperatures below 50° C.
In the case of normal (i.e. non-ionic) polyesters, there are no sites for ionic interaction with ionic dyes. Accordingly, normal polyesters may only be dyed by disperse dyes. Compared to ionic dyes, disperse dyes have smaller molecular extinction coefficients (i.e. less intense colorations) and lower build-up properties. Therefore, these dyes cannot impart bright and deep colors. Moreover, fastness to sublimation and wet treatments of disperse dyes are relatively poor compared to other classes of dyes.
Cationic dyeable polyesters are generally prepared by the co-polymerization of an isophthalic acid component containing a sulfonic acid group, which makes it possible to use cationic dyestuffs for polyester staple fibers and filaments. Generally, the sodium salt of 5-sulfo-isophthalic acid (Na-SIPA) is used as a cationic co-monomer. The cationic dyestuff may contain amino groups, ammonium groups, or quaternary nitrogen-heterocyclic groups.
Dyeing of CD-PET (cationic dyeable polyethylene terephthalate) is conducted using ion exchange methods. In such methods, sodium cations (Na+) from the CD-PET are substituted with bigger cations. Thus, polyethylene terephthalate is chemically modified in a manner that cationic dyestuffs can form a chemical complex with the fiber.
The chemistry of producing CD-PET, however, has traditionally been complicated because of the acidic character of Na-SIPA (sodium κ-sulfo-isophthallic acid), especially in connection with hydrolytic or glycolytic conversion. Therefore, after direct addition of this salt at the polyethylene terephthalate esterification stage, diethylene glycol (DEG) formation reaches high levels because ether formation is acid-catalyzed. Additionally, the acidic character enhances the TiO2 agglomeration. The result is difficulty in spinning fibers, and a low melting point of CD-PET. Attempts have been made to avoid these problems by adding bis(hydroxyl ethyl) isophthalate-5-sulfonate at the end of esterification step. This is separately prepared by using the cationic salt monomer 5-sulfoisophthalic acid (SIPA)/dimethyl-5-sulfoisophthalate (DMSP) with monoethyleneglycol (MEG). As a result, diethylene glycol formation is minimized but issues related to spinability remain, primarily due to the formation of trimers, tetramers, and oligomers during the preparation of bis(hydroxyethyl)isophthalate-5-sulfonate.